1. Field of the Invention
The present invention relates generally to the field of two-wire, high frequency electronic ballasts for powering gas discharge tubes and the like and, more particularly, to a simplified two-wire electronic ballast arrangement which eliminates inductance external to the transformer and allows wide-range dimming.
2. Description of the Prior Art
Typical fluorescent tubes comprise a sealed cylinder of glass having a heating filament at either end and filled with a gas such as mercury vapor. The supplied voltage is utilized to heat the filaments to a point where a thermoionic emission occurs such that an arc can be struck across the tube causing the gas to radiate. Initial radiation given off by gases such as mercury vapor is of a short wavelength principally in the ultraviolet end of the spectrum and thus little visible light is produced. In order to overcome this problem, the inside of the tube is coated with a suitable phosphor which is activated by the ultraviolet radiation and, in turn, emits visible light of a color that is characteristic of the particular phosphor or mixture of phosphors employed to coat the tube. An important consideration in the operation of such fluorescent tubes is concerned with the fact that in order to sustain the arc across the tubes, the filament voltage must be maintained to a predetermined level. It is maintaining this predetermined voltage level and, at the same time, reducing the cost of components required to do so which poses the greatest problem in devising a scheme for dimming the output of the fluorescent tubes in a solid state ballast system to produce an energy-saving, light-dimming arrangement.
Solid-state ballasts must provide the same primary function as the conventional core-coil ballasts well known in the art, i.e. they must start and operate the lamp safely. Solid-state ballasts normally convert conventional 60 Hz AC to DC and then invert the DC to drive the lamps at a much higher frequency. That frequency generally is in the 10 to 50 KHz range. It has been found that fluorescent lamps which are operated at these higher frquencies have a higher energy efficiency than those operated at 60 Hz, and they exhibit lower power losses. In addition, at high frequencies, annoying 60 cycle "flickering" and ballast hum are eliminated.
Prior art electronic ballasts normally employ current fed inverters which require a transformer with a separate inductor and tuning capacitor in the primary circuit to obtain the proper tuned high frequency sine wave output. The inductor or choke coil normally has a ferrite core and is required to prevent the current to the transistor inverter from changing at the high 30 KHz inverter frequency so that an almost constant current is switched between the two transistors. The current waveform through them is trapezoidal rather than one exhibiting a high peak thereby keeping the transistor collector-to-emitter saturation voltage low. The choke coil also has a high impedance at the high inverter circuit frequency which helps reduce radio-frequency noise coupled to power lines.
These prior art devices, while somewhat successful, also have several drawbacks. The choke coil is an important functional part which is necessary to produce a high frequency tuned sinusoidal input in such prior art devices. However, it is an extremely costly element of the electronic ballast. Also, no practical low-cost method of dimming such circuits exists in the prior art.